Materials of Construction for the CPI

August 1, 2023 | By William M. (Bill) Huitt

The proper selection of materials of construction is crucial. The example presented here shows how to use standards to help narrow down the options

Since the late 1800s, a steady body of pressure technology codes and standards have been under development (see box on p. 36) providing engineers with standardized methods to safely design and fabricate pressure vessels and other pressurized process equipment used in the chemical process industries (CPI). This article gives an overview of the relevant standards, and presents an example of how they can be used to narrow down the selection of suitable materials of construction (MoC).

Operating for 125 years, ASTM International (ASTM; West Conshohocken, Pa.; www.astm.org) has come a long way since standardizing on the material and testing of railroad track steel. It currently publishes and maintains 12,500 standards globally in over 140 participating countries with the help of more than 30,000 volunteer and staff members. This is an organization that we depend on for determining and evaluating a material’s chemistry, its strength, its testing, and manufacturing requirements. ASTM is also involved in developing testing methodologies and procedures.

In all, ASTM publishes, among other things, six different types of standards that include the following:

Test Method Standards: A set of procedures that provide instruction, guidelines, and parameters necessary to acquire and analyze sample evidence of fluids and materials.

Practice Method Standards: Are instructions on the recommended approach to achieving a deliverable in a standardized fashion. Such examples include F748 Standard Practice for Selecting Generic Biological Test Methods for Materials and Devices and E1816 Standard Practice for Measuring thickness by Pulse-Echo Electromagnetic Acoustic Transducer (EMAT) Methods.

Specification Standards: Are standards that specify what the intended use is for a specific material, the material’s chemical composition parameters, mechanical properties, the manufacturing process, testing requirements, and all other use and manufacturing requirements pertaining to a specific material and its various product forms

Classification Standards: Provide guidance and requirements for the process of assigning the various materials, services, or systems into their proper category. These requirements may relate to the origin, the physical properties, or the chemical properties of the product itself.

Guide Standards: Are collections of information or series of options that do not recommend specific courses of action. They generally inform people of the knowledge and approaches being taken in given subject areas.

Terminology Standards: Provide definitions of terms and explanations of symbols, abbreviations, and acronyms used throughout ASTM.

What we can pull from the above list, relative to this discussion, is item three, Specification Standards, which covers the following seven categories:

We will further narrow our focus down to the above Class A – Ferrous iron and steel metals and products. This is where most of the piping and equipment material for the CPI are found. Nonferrous materials are frequently used as well, but that would be too much to cover here. ASTM includes plastics under the above Class D – Miscellaneous materials. So, for nonmetallic (NM) material, refer to the relatively new ASME NM standards first published in 2018 by the American Society of Mechanical Engineers (ASME; New York, N.Y.; www.asme.org). They include the following:

In the process of selecting material for pressurized equipment, there may be some confusion surrounding the issue of specification numbers between ASTM and those of ASME having a different prefix. What I refer to is the prefix A attached to ASTM specification numbers and the same ASME material specification numbers in the ASME Boiler and Pressure Vessel Code (BPVC) Section IIA for Ferrous Metals having an SA prefix. Section IIB for Nonferrous Material will have SB as the prefix. Within the BPVC are listed materials that comply with applications used in the manufacture of boilers and pressure vessels. These listed materials are contained in the ASME BPVC Sections II Materials, as follows:

ASTM, in standardizing a material specification, will provide all of the essential elements based on a material’s chemistry, manufacturing process, intended use, testing and examination requirements. And in many cases, a specification will be broken down into multiple grades and classifications, plus various supplemental qualifications.

ASME BPVC draws from the ASTM pool of material specifications and then qualifies, or vets those specifications through testing and theoretical analysis to qualify the material for use under the requirements of the BPVC. In doing so, the vetted material specification will retain the same ASTM number, but ASME will add an “S” to the ASTM prefix to designate the adopted ASTM specification as an ASME qualified specification. For example, ASTM A182/A182M would become ASME SA182/SA182M under the ASME BPVC. In many cases the adoption of an ASTM material specification will remain the same, without change. But in some cases, the ASME material specification will be modified from its original ASTM material specification to meet the requirements of the ASME BPVC. In the ASME version of a material specification, it will be explained on the cover page of a material specification as to whether or not a material specification was altered in any way, as shown in Figures 1a and 1b.

FIGURE 1. Shown here are the covers for SA-178/SA-178M (a), which indicates no change from the ASTM version, and (b) for SA-182/SA-182M, which indicates a change from the ASTM version

Two things you may notice, besides the indicated note and the different specification numbers, is that Figure 1a has the ASTM logo included on the specification cover page in which there is no change to the specification. This specification has been adopted by ASME as written by ASTM. And it states so on the cover sheet in posting that it is, “Identical with ASTM Specification A178/A178M-95.”

In Figure 1b, the ASTM logo is not included on the cover page, due to the fact that ASME has determined that the specification required modification in order to meet the requirements of the BPVC. And it states so on the cover sheet in posting that it is, “Identical with ASTM Specification A182/A182M-18 except for the inclusion of Grade F316Ti in paragraph 7.3.1, and the removal of reduced strength levels for thicker sections of Grade F53 in Table 3 and note G.”

With regard to piping, the materials listed for use in piping applications are listed in Appendix A-1 and A-1M of the ASME B31.3 Process Piping Code. In the listing of acceptable materials under the piping code, you will notice that the ASTM material specification number is retained, as in ASTM A182/A182M. The materials listed in B31.3 have been vetted by that committee and approved for use as pressure-containing piping material without change to the ASTM specifications. Any material not listed in B31.3 for use in pressure-containing fluid services can be submitted to the committee for approval in accordance with ASME B31.3, paragraph 323.1.2, as follows:

323.1.2 Unlisted Materials . Unlisted materials may be used provided they conform to a published specification covering chemistry, physical and mechanical properties, method and process of manufacture, heat treatment, and quality control, and otherwise meet the requirements of this Code. See also ASME BPVC, Section II, Part D, Appendix 5. Allowable stresses shall be determined in accordance with the applicable allowable stress basis of this Code or a more conservative basis.

By stating the following in paragraph 323.1.2 above: “…a published specification covering chemistry, physical and mechanical properties, method and process of manufacture, heat treatment, and quality control…” it is referring to internationally accredited material specification organizations. such as the following:

Working across international boundaries with specifications published by organizations like those listed above, can be both a little daunting, confusing, and a little difficult at times. There is no intended direct correlation between the various material specifications and standards published around the world. But in these many material specifications, the chemistry is typically not specified in exacting amounts. The chemical compositions are instead given as a weight percentage with a maximum value or a value range, not a specific target amount. What this does is provide an environment or situation in which the chemistry in a specification from one country can overlap that of another country. But the practicality of an exact chemistry match occurring, in which all of the chemistry constituents between two specifications from two different countries will match up, is not going to happen. Transposing specifications from those published by two different countries requires someone very familiar with the nuanced effect that the various alloys have on the chemistry that make up a steel. This also applies to the melting temperature, the cooling process, and post treatment of the metal manufacturing requirements. Such determination in transposing material specifications published by one organization to that of another requires an expertise in metallurgy.

With the wide and varied selection of steels that are available in the marketplace, how do you go about determining which material to use for a specific application and fluid service? There are many more variables that go into determining a proper material for any number of specific process applications than are discussed here. But what we can do is set the stage for a methodology by providing examples of things to be aware of. And I say that because it is not merely a matter of matching up a metallic material to a process fluid service. There are outlier conditions that may also need to be considered. And you will see why as we move through this discussion.

Finding a compatible material to contain a process fluid service for piping or equipment is a paring down process of elimination. The following outlines key points in a process that helps to arrive at a suitable MoC:

For Step 1 indicated above, what I mean by “reasonable” is to disregard, right at the start, the more costly materials, such as the higher alloy materials. Begin with the basic carbon steels such as an ASTM A53 or A106 if they are a good place to start from. The process might then lead you to a possible stainless steel, nickel alloy, pipe lined with nonmetallic material, or nonmetallic material. So, in order to work through the above four steps in some meaningful way, we will set up a sample fluid service to make some necessary points in our evaluation. The sample fluid service will consist of the following essential elements:

The first step is to find compatible material types for a specific process fluid service. This will immediately pare down the overly broad field of possible candidate materials to a smaller select group of materials. To do this I would suggest referring to sources that have compiled test results data on the compatibility of various steel materials (as well as other metals) with that of various fluids.

Such sources will provide the rate of corrosion a material will sustain while in contact with a specific fluid at various temperatures. They typically present the results by indicating, in various ways, that the material is acceptable, nominally acceptable, poor, or not recommended for application with the specified fluid at a specified temperature.

In selecting our material in this regard, we need parameters upon which we will make our decision in selecting a suitable material. Those parameters include the degree of corrosion rate we can accept for the intended service application. So before starting through the selection process I will define and explain those parameters.

To begin with, the design basis of a project is a procedure that lays out the essential elements of designing and constructing a process facility. In it, along with many other foundational metrics for a project, will be a durational period for which the process facility is intended to operate and the basis from which operational cycles are calculated and the length of time in service for materials is established. Knowing that length of time permits us to determine, once we know the corrosion rate of a material, how much wall thickness material loss will possibly occur over the life of a piping system or a pressure vessel. This gives the wall thickness that will be required in order to allow for the loss of material through corrosion over the expected facility life and still have sufficient strength in the remaining wall thickness to retain the specified internal pressure at the end of a facility’s lifecycle.

In essence, what needs to happen in developing specifications for a project, or for a corporate library of procedures, specifications and standards, is to give some forethought with regard to corrosion allowance (CA). This is an essential element in the process of selecting material. But in building a material specification, it should be created in a way in which any number of conditions (fluid services versus MoC) can be met. This means that an attempt should be made in developing a specification to allow it to meet the needs of multiple fluid services, if possible.

In this generalization we will look at corrosion rates for carbon and non-alloy steels for non-aggressive fluids as well as stainless steels and other alloys selected for use against aggressive fluids, as follows:

Example 1.

a. Corrosion rate rule of thumb for non-aggressive fluids (that is, cooling tower water, chilled water, and so on) in carbon-steel pipe is to allow a corrosion rate of 1 mil/yr (0.001 in./yr) of general corrosion. This is a nominal value to allow for the rate of corrosion for this general type of material. If the rate of corrosion exceeds this to a point in which the pipe wall schedule has to be increased, then you might need to look at an alloy steel.

b. A corrosion rate for stainless steels and higher alloys selected for use against aggressive fluids (that is, sulfuric acid, NaOH, hydrofluoric acid and so on) is typically assessed at 0.000 in.

c. Assume a design basis of a 20-year facility life cycle.

d. The amount of corrosion allotted for carbon-steel material, based on 1 mil/yr over the 20-year life of the facility would be 20 x 0.001 in. = 0.020 in. (total expected corrosion over the life of the facility)

e. The amount of corrosion allotted for alloy-steel material, over the 20-year life of the facility would be 0.000 in., but the specified CA would, in some cases, be 0.030 in. to include a margin of safety.

f. Corrosion seldom occurs evenly throughout a system. It would therefore be wise for maintenance within the operating facility to periodically check for localized corrosion at specific locations, such as impingement areas, possible cavitation areas, downstream from pressure-reducing valves (PRVs), and so on.

Based on the suggested corrosion rate expressed in item d above, the CA typically applied to carbon steel would be 0.050 in. And based on the suggested corrosion rate expressed in item e above, the CA typically applied to steel alloys is 0.00 in. to 0.032 in. The reason behind those applied CA values will be made apparent after the following example listing the essential elements of a selected material:

Example 2.

a. 6 in. NPS A53 Gr B ERW carbon-steel pipe

b. Sch. 40 has a wall thickness = 0.280 in.

c. Deduct manufacturers tolerance of 12.5% from specified wall thickness (the manufacturing tolerance is specific to each material specification)

1) Manufacturers tolerance 0.125 × 0.280 = 0.035 in.

d. Assigned CA value is deducted from specified wall thickness:

1) CA typically used for carbon steels = 0.050 in.

e. Total deductions from wall thickness = 0.035 in. + 0.050 = 0.085 in.

f. 0.280 in. wall thickness. – 0.085 in. total deductions = 0.195 in. remaining wall thickness at the end-of-life cycle

Projecting a remaining wall thickness of 0.195 in. at the end of the calculated life span of a facility the burst pressure, based on that remaining wall thickness, would be calculated using Equation (1):

Where:

B = Burst pressure, psi

D = Pipe O.D., in.

S = Minimum ultimate tensile strength, psi

T = Pipe wall thickness,

For this example, Equation (1) gives the following:

B = (2 × 0.195 × 60,000)/6.235 = 23,400/6.235 =

3,753 psi (≈ 3,750 psi)

With this information, we can then calculate maximum allowable pressure (MAWP) for the piping based on its lifecycle of 20 years. MAWP is a term used in the ASME BPVC and is not used in the piping code. But using it for piping in this case will help the narrative. Using a safety factor of 4, and a bursting pressure of, say, 3.700 psi, the MAWP is 3,750/4 = 937.5, or approximately 930 psi (refer to ASME Sect. VIII, Div 1, para. UG-101, (m), Bursting Test Pressure (with the exception that the equation used above to calculate the safety factor does not represent the full equation found in paragraph (m), (refer to Equations (2) and (3)):

or

Where:

E = efficiency of welded joint, if used (see Table UW-12)

P = Maximum allowable pressure, psi

Sµ = specified minimum tensile strength at room temperature

Sµ avg = average actual tensile strength of test specimens at room temperature

Sµr = maximum tensile strength of range of specification at room temp.

Having posited an approach above in determining the strength of our piping system at the end of its lifecycle, we will now revisit the manner in which the assigned CA was determined for both the carbon-steel materials and the alloy-steel materials.

Item “a” under Example 1 above, points out that the corrosion rate allotted for carbon steel material in this discussion is based on 1 mil/yr. It then goes on in item “d,” under Example 1, to extrapolate that corrosion rate out over a 20-year facility life span as in: 20 × 0.001 in. = 0.020 in. The assumption here is to conclude that, if you need to assign a higher corrosion rate than 0.001 in./yr, then you may need to go to an acceptable steel alloy, nonmetallic material, or pipe lined with nonmetallic material. But how do you know how much of a corrosion rate to assign to a particular fluid service as compared to a specific MoC?

As mentioned earlier there are published resources available that can provide such data. In one such publication, you will find the legend in Table 1 to indicate rates of corrosion.

The symbols in Table 1 will indicate in Tables 2 and 3 the expected corrosion rate per year based on our fluid service and the data from the sources mentioned earlier, namely, fluid 50% NaOH, 70°F operating temperature, 95°F design temperature, 80 psig operating pressure and 110 psig design pressure.

In checking the published data for 50% NaOH with a design temperature of 95°F and a MoC of carbon steel, we find the expected rate of corrosion information given in Table 2. Refering to Table 2, we see that the 50% NaOH solution at the 95°F design temperature in carbon steel, line item number 6 within the table would indicate a possible corrosion rate of between 0.002 to 0.020 in./yr. Following the old adage that you should, “plan for the worse and hope for the best,” we will assume a corrosion rate of 0.020 in./yr, which gives us an accumulated corrosion loss of 0.400 in. at the end of the 20-year facility lifecycle. With that rate of corrosion, 0.020 in./yr, it would surpass both the manufacturing allowance (0.035 in.) and the specified corrosion allowance within 5 years, which is not even close to the desired 20-year lifecycle.

I mentioned earlier that, “There are outlier conditions that may also have to be considered.” And at this point I will include one. If you are heat-tracing a piping system containing 50% NaOH in which the heat tracing is set at somewhere between 250°F and 455°F, the heat tracing will create a high-temperature zone on the piping. In referring to Table 2, we can see that once the pipe-wall temperature exceeds 200°F with piping that contains 50% NaOH, it has a detrimental effect on the piping material. Depending on how much in excess of the 200°F the heat tracing is operating at, pits will begin to form. These corrosion pits will penetrate the pipe wall within a matter of a week or two and leak the NaOH out to the environment. This is intended to point out the fact that there could be extenuating circumstances to consider beyond the rudimentary fluid compatibility at design temperature.

The first consideration of carbon steel was based on cost and the possibility that it would be acceptable, which in this case, it does not. If we go to the next higher cost value, we might look at a 304 stainless steel. It is one of the less expensive steel alloys that might very well do the job. In looking at Table 3, 304 stainless steel is very compatible with the 50% NaOH fluid service at a design temperature of 95°F. And the stainless material remains compatible with 50% NaOH at elevated temperatures up to 212°F.

In this case, a schedule 10S 304 stainless steel would be a good selection, from both a cost and compatibility standpoint for the 50% NaOH fluid service. But there are other cost-wise options as well, such as nonmetallic material. But the point being made here is in the selection process itself. And in selecting a compatible MoC ensure that you have identified any outlier concerns, like the heat tracing in this case, or make certain that there are no additional concerns. A conversation with the chemical engineer would help along those lines. Such a misstep could have catastrophic implications weeks or even years down the road — a misstep that could be lethal.

In addition to the outlier concern regarding heat tracing of an otherwise ambient fluid temperature described in the above discussion, ASME B31.3 Appendix F, paragraph F323.1 General Considerations, points out a number of other concerns that might be considered when selecting piping material, as follows:

And with regard to the selection of material for systems that fall under the rules for ASME B31.3 High Pressure piping, things become much more complicated. For a high-pressure system, you will not only take into consideration material compatibility with the fluid service, but will also need to consider the effect of high pressure, we’ll say, as an order of magnitude, in excess of 1,000 psig, on the material itself in conjunction with the design temperature.

At higher pressures or elevated temperatures, or both, you will need to assess the risk to the selected material for such concerns as hydrogen embrittlement as well as items of concern listed in ASME B31.3, paragraph K302 Design Criteria for high pressure piping. Those concerns include the following:

And while the material selection process, as discussed, pertains to a piping system as an example, it can also be applied to process vessels and other equipment, with one glaring exception. The rules for pressure vessels are much broader and much more specific and demanding than the rules for piping. The dynamics that have to be considered when selecting material for equipment are different, and the fabrication rules are much different and varied than they are for piping.

The last thing to be considered will be the installed cost of the selected MoC. The installed cost is the last thing to be considered because the safety and integrity of a process system is first and foremost in the selection process. And once the selection has been decided upon for compatibility, safety and integrity, and you are left with multiple materials, including nonmetallic materials, it then comes down to the installed cost.

Including the installed cost can actually move the needle from a low front-end cost for a material that may be more costly to install to a material that has a higher front-end cost, but may be less costly to install. So, when making that final decision on which material to go with, base it on the total installed cost, not the base material alone.

The material selection process discussed in this article is explained in a rather straight-forward and simplified manner, but the process itself, and all that it entails, in all actuality should be carried out by an individual with years of experience in the CPI — someone with a history of working with the various materials and fluid services used in this industry.

Edited by Gerald Ondrey

William M. (Bill) Huitt has been involved in industrial piping design, engineering and construction since 1965. Positions have included design engineer, piping design instructor, project engineer, project supervisor, piping department supervisor, engineering manager and president of W. M. Huitt Co. a piping consulting firm founded in 1987 (1070 Sarala Road, St. Louis, MO 63131-0154; Email: [email protected]; Website: www.wmhuittco.com). His experience covers both the engineering and construction fields and crosses industry lines to include petroleum refining, chemical, petrochemical, pharmaceutical, bioprocessing, pulp-and-paper, nuclear power, biofuel and coal gasification. He has written numerous specifications, procedures on design and construction, guidelines, papers and magazine articles on the topic of piping design and engineering. Huitt has also written “Bioprocessing Piping and Equipment Design — A Companion Guide for the ASME BPE Standard,” published through the publishing partnership of ASME Press and Wiley Publishing. He is a past member of the International Society of Pharmaceutical Engineers, the Construction Specifications Institute and a current and active member of ASME. He is a member of the B31.3 section committee, past Chair of B31.3 Subgroup H on High Purity Piping, Vice Chair of ASME BPE subcommittee on Certification, a member of three other ASME- BPE subcommittees and is active on several Task Groups. Huitt is also a member of the ASME Board on Conformity Assessment for BPE Certification, a member of the A13 Standards Committee for Standard A13.1 Scheme for the Identification of Piping Systems, a member of the API Task Groups for RP-2611 and RP-1110. He also authored the training program and provides training to ASME consultants for auditing fitting manufacturers who are applying for or renewing their ASME BPE Certification.